This invention relates to a device for generating mechanical oscillations and to a method for calculating the resonance frequency of such a device.
Assessment of the service life of test objects by fatigue tests is known. To do so, the test object is excited with a resonance frequency of the test object. Excitation is achieved by means of a device for generating mechanical oscillations or by means of an oscillator including a piezoelectric excitation system and two masses coupled by it. The test objects on which fatigue tests are conducted are for example engine components, e.g. stator vanes or rotor blades of a turbine.
To ensure that a test object is excited in a resonance frequency, it is required that the oscillator also oscillates in the resonance frequency of the test object. To do so, the natural frequency of the oscillator must match that of the test object. Additionally, the AC voltage signal applied to the oscillator must have this frequency.
Piezoelectric excitation systems are based on the inverse piezo effect and are used, among others, in the field of fatigue tests. The general setup includes two masses bolted to one another as well as piezo elements arranged between them, said masses being subjected to a mechanical pretension. One of the masses is used as the yoke in which the test object is clamped. The natural frequency of a piezoelectric excitation system of this type is dependent on the piezo material of the piezo element and on the masses involved. The natural frequency is thus determined by the design constraints of the piezoelectric excitation system. To allow the generation of differing resonance frequencies for differing test objects, a plurality of piezoelectric excitation systems each having a differing natural frequency must be kept available.
It must be regarded as a disadvantage that the known piezoelectric excitation systems can each serve only one natural frequency or a narrow band of natural frequencies, and that a plurality of these piezoelectric excitation systems must be kept available in order to cater for differing resonance frequencies. There is a need for a device to generate mechanical oscillations which can be set in respect of its natural frequency and accordingly used to generate resonances of test objects in a wide frequency range.